The new and fascinating observation has been made that the pituitary hormone prolacting (Prl) accelerates galactose-induced cataractogenesis. In the development of this type of cataract, there is lenticular water and electrolyte imbalance as well as a transient increase in mitosis in the lens epithelium; Prl is known to affeft electrlyte balance in a number of species and has been reported to have some control on lenticular mitosis in amphibians. Because of these correlations and the results of pilot studies, experiments have been designed to investigate the mechanism(s) by which this hormone, previously unknown to affect cataractogenesis, influences the cataractic process. These experiments wiull focus on the effect of Prl on electrolyte and water balance, mitosis, cataract morphogensis and cataract regression in the lens of the galactose-fed rat. Techniques used will include light microscopy, autoradiography, and atomic absorption spectrophotometry. Since Prl is a substance common to all vertebrates including humans, the outcome of these experiments will provide new insight into mechanisms underlying human and experimental cataractogenesis. This completely new avenue of investigation requires additional data before more extensive studies are undertaken.